Tyranids were my first army and I can field 1 First Curse formation. I've wondered how many would fit into 1850 and what to bring along if there was room. The following from Genestealer Cults perfectly fits 1850.

--CAD--HQ2 Magus @ ML 2, one with the Crouchling, other with the Staff

Troops10 Neophytes, stock10 Neophytes, stock

--Formations--4x First Curse each with a ML 2 Patriarch

Subterranean assault could get each of the First curse formations close & possibly even charging on the first turn.Pick a Patriarch as your warlord and it's another chance on a six to decide on one formations deployment.

The CAD Magus can grant even more supporting psy dice and spells. This army would have 12 psy dice, so casting wouldn't be an issue until later in the game and some casualties have set in. However the auto pass Look Out Sir would help keep the key models alive.

The Magus could hopefully get the summoning spell; providing new units to hot swap into if the current one is getting low on models. They could do this for the Patriarchs too, 3 WC summons has the option for 8 Purestrains.